Known from the patent specification DE 199 53 317 C1 is an inhalation nebuliser of the type described above. The aerosol membrane generator described therein comprises a cylindrical liquid storage container which is delimited at one end face by a membrane having the shape of a circular disc.
A liquid disposed in the liquid container touches the side of the membrane facing the liquid storage container. DE 199 53 317 C1 furthermore describes an oscillation generator, for example a piezo crystal, which surrounds the membrane in a circular manner and is connected thereto such that the membrane can be caused to oscillate by means of the oscillation generator and an electric drive circuit.
The liquid abutting the membrane on the one side is conveyed through holes in the oscillating membrane to the other side of said membrane and is emitted on this side into the mixing chamber as an aerosol.
Known from the utility model DE 295 01 569 is an ultrasonic liquid nebuliser having a piezo crystal which is caused to oscillate electrically by an oscillator circuit, said oscillator circuit being supplied by a power supply device.
DE 295 01 569 describes an oscillator circuit which comprises a current limiting circuit and which is connected with an electronic temperature limiting circuit that compares a temperature-dependant electric signal occurring at the piezo crystal in a threshold circuit, the comparison signal of which activates a bistable circuit which blocks the oscillator when a limiting temperature in the piezo crystal is reached.
The disclosure of DE 295 01 569 is thereby directed at a protective mechanism for an ultrasonic liquid nebuliser in which the piezo crystal itself causes the liquid to oscillate and is in contact with a comparatively large amount of liquid. The liquid nebuliser described in DE 295 01 569 must furthermore accordingly use large currents in order to cause the large amount of liquid to oscillate.
Constant contact between the piezo crystal and the liquid is necessary owing to these large currents and the resulting large temperature differences in order to prevent destruction of the piezo crystal. If there is no longer any liquid present, the piezo crystal heats up very quickly and is destroyed if the oscillating circuit driving the nebuliser is not switched off immediately.
Only much smaller currents flow in inhalation nebulisers of the type described at the beginning of this document, i.e. in inhalation nebulisers having membrane aerosol generators, and therefore only comparatively small temperature differences occur. In such inhalation nebulisers, the lack of liquid does not directly lead to heat-related damage to the piezo-electric elements. However, if a membrane inhalation nebuliser runs without a load, this can, on rare occasions, cause the membrane to break.
However, it is also necessary in inhalation nebulisers having a membrane generator to reliably detect the presence of a liquid to be nebulised. This is because, on the one hand, the basis for a very high dosage accuracy is thereby created and, on the other hand, it is possible to reliably indicate the end of a therapy session to the patient. Furthermore, by immediately disconnecting the inhalation therapy device, it is possible, for example, to save a battery.
Therefore, the use of a protective mechanism such as described in DE 295 01 569 is not necessary in inhalation nebulisers of the type in question here and is not possible either owing to the much smaller currents and temperature alterations.